Recently, thorough research into use of heavy oil having high aromatic content while containing large amounts of heteroatoms such as sulfur and nitrogen is ongoing, and also the demand for a middle distillate which is the feed for transport fuel is increasing. Especially, hydroprocessing in a refining process, including hydrogenation, hydrodesulfurization (HDS), hydrodenitrogenation (HDN), etc. is regarded as important.
As the catalyst useful for such hydroprocessing, there is required to develop a catalyst which exhibits high hydroprocessing activity even in the presence of an impurity such as sulfur (acting as a catalyst poison), and also suppresses C—C bond cleavage (hydrogenolysis) to thereby inhibit production of hydrocarbons having comparatively low value due to a decrease in the number of carbons. The catalyst for hydroprocessing may include molybdenum sulfide such as NiMo, CoMo, etc. and a precious metal such as platinum (Pt), palladium (Pd), etc., which are currently widely used. In this regard, a molybdenum sulfide-based catalyst is known to have lower activity but to be more resistant to sulfur, compared to a precious metal-based catalyst. On the other hand, the precious metal-based catalyst shows high activity in the absence of sulfur but suffers from being rapidly deactivated in the presence of sulfur.
Meanwhile, in order to suppress hydrogenolysis during hydroprocessing, formation of an alloy such as Pt—Sn, Pt—In, etc. (F. B. Passos et al., J. Catal., 160:106, 1996), or partial poisoning of the surface of metal with a catalyst poison such as sulfur (P. Govind Menon, Eng. Chem. Res., 36:3282, 1997) has been studied. However, such methods are problematic because molecular sulfur has to be continuously added to the feed and byproducts may be generated by the added sulfur.
An alternative to the method of overcoming limitations of the conventional precious metal-based hydroprocessing catalyst, for example, supporting of metal particles in microporous (pore diameter of less than 1 nm) zeolite to thus enhance stability is under study. Specifically, C. Song et al. have researched hydrogenation of naphthalene in the presence of a sulfur compound after supporting of Pt in mordernite type zeolite (C. Song et al., Energy & Fuels, 11:656, 1997). It was reported that mordernite has two cages with different sizes, wherein hydrogenation is carried out only on Pt supported in the cage having a large size to which an organic molecule is accessible, and only hydrogen may be selectively diffused and activated on Pt supported in the cage having a small size, so that the activated hydrogen atom may move to Pt supported in the cage having a large size through a spillover phenomenon to thereby suppress deactivation of metal. However, as hydrogen sulfide (H2S) produced upon decomposition of a sulfur compound diffuses into the cage having a small size, the metal in the cage having a small size may also become deactivated, and thus the effects thereof are limited.
In a study conducted by Hong Yang et al., A-type zeolite having Pt supported therein was used as a catalyst for hydrogenation of naphthalene (Hong Yang et al., J. Catal., 243:36, 2006; US Patent Publication NO. 2009/0048094). Specifically, the catalyst was designed such that, as a result of decreasing the final pore size of a zeolite cage up to about 2.9˜3.5 Å by incorporating precious metal nanoparticles in the zeolite cage and then performing post-treatment (CVD, CLD, cation exchange or combination thereof), only molecular hydrogen may pass through the pores but an organic sulfur molecule (H2S having a kinematic diameter of 3.6 Å) cannot pass through the pores to thereby suppress contacting of the precious metal component with the poisoning material, that is, the sulfur compound. By means of such a catalytic structure, activated hydrogen (i.e. dissociated hydrogen) by precious metal undergoes spillover through the zeolite pores to thus induce hydrogenation, and may recycle the catalyst therearound (the sulfur compound which poisoned catalyst sites is removed by hydrogen). Furthermore, such researchers have made attempts to decrease the pore size by supporting Pt in A-type zeolite and then performing ion-exchange with K+ and coating with silica. It was reported that the catalyst thus synthesized enables naphthalene, which cannot diffuse into zeolite pores, to be successfully hydrogenated through spillover of activated hydrogen, and is very resistant to sulfur.
In a study conducted by Chen et al., hydroprocessing reactivity for actual crude oil reactants containing diverse sulfur compounds was measured using P-type zeolite having Pt supported therein, in which P-type zeolite has a smaller pore size than A-type zeolite (Song Chen et al., Proceedings of the World Congress on Engineering and Computer Science, vol. 2, 2010). This paper proposed the catalyst to be configured such that Pt is encapsulated in the sodalite cage of P-type zeolite on the basis of shape selectivity and hydrogen spillover principle. In this case, H2 passes through the pores, whereas H2S does not. Hence, even when such a catalyst is exposed to a high sulfur-containing environment, hydroprocessing activity thereof may be maintained.
Despite the above research results, there are still needs for hydrogen spillover-based catalysts which exhibit further improved catalytic activity in the art.